This invention relates to the art of geophysical prospecting in general, to the art of radioactivity well logging in particular, and more particularly, to improvements in such logging wherein the lifetime of neutrons in the formation is measured.
It is known in the prior art to irradiate the formations for a period of time sufficient to activate an appreciable number of nuclei of material in the formations, thereby rendering the material artificially radioactive, and at an interval of time following the cessation of the radiation to measure the artificial radioactivity. The time is related to the half-life of the activated material and is of the order of minutes or seconds for each of the common elements of the earth as are activated appreciably by this means.
In U.S. Pat. No. 3,379,882 to Arthur H. Youmans, assigned to the assignee of the present invention, there is described a system for radioactivity well logging wherein the formations are irradiated with neutrons from a periodically varying source operating at a repetition rate of the order of magnitude of hundreds or thousands of cycles per second, being thus alternately on and off for periods of hundreds of microseconds. A detecting system is synchronized with the source to operate while the source is off.
Furthermore, in U.S. Pat. No. 3,706,884 to Arthur H. Youmans, also assigned to the assignee of the present invention, there is described a system for using three detection gates following each neutron pulse and associated circuits for substantially eliminating radiation background count in pulsed neutron well logging. Although the system described in this patent has met with a high degree of success, there has sometimes been a problem when using the three-gate system because of statistical inaccuracies.
Furthermore, in U.S. Pat. No. 3,566,116 to William B. Nelligan, especially with regard to FIG. 6 of that patent, means are provided for pulsing a neutron generator three times followed by the omission of a single neutron pulse in an attempt to measure the background radiation. However, this particular system, by skipping every fourth neutron pulse, does not recognize the problem associated with disabling the pulse generator so often with its concomitant loss of neutrons. Furthermore, with such a system, the pulse generator is not likely to be disabled for a period sufficient to allow all the background radiation to die away.
Furthermore, in U.S. Pat. No. 4,041,309 to Eric C. Hopkinson, assigned to the assignee of the present invention, there is described a system of pulsed neutron logging which provides means for pulsing a neutron generator for a considerable number of times and for allowing the generator to be off for a small percentage of time compared to the number of times the generator is pulsed in order to measure the background radiation while having no substantial effect on the number of neutrons generated.
Thus, the prior art recognizes that the use of gamma ray detectors in pulsed neutron logging may on occasion necessitate a background correction. Almost all stationary measurements will require such a correction. One means of drastically increasing the observed count rates, and thus better repeatability, involves the use of a low energy discrimination of the detected gamma rays. However, such a system involving low discrimination has a background component. Thus, a better logging system results when an adequate system to account for the background is provided.
As with the prior art systems, the present invention involves count rates measured during three intervals of the decay period. However, the present invention comtemplates that the background is calculated by the following equation: ##EQU3## After the background (B) is computed, the macroscopic absorption (.SIGMA.) is calculated using gates 1 and 2 (N.sub.1 and N.sub.2) and B by the equation: ##STR1##
It should be appreciated, moreover, that in 1973, in a paper presented in Poland by Edward Chrusciel, Jerzy Massalski, Krzysztof Morstin, and Antoni Starzec, at the Institute of Nuclear Techniques, Krakow, Poland, that the background level was indicated to be determined as follows: ##EQU4## However, as pointed out within that paper, the authors continued to determine .SIGMA. by a relationship which involved the expression ##EQU5## which is identical to that used in the U.S. Pat. No. 3,706,884. Even though having an expression for the background as used in the present invention, they did not recognize how it could be used to increase the statistical accuracy of the system.
It is therefore the primary object of the present invention to provide an improved apparatus for determining geophysical characteristics of formations surrounding boreholes;
It is still another object of the invention to provide improved means for computing and/or eliminating the effect of background radiation upon pulsed neutron logging data.
The objects of the invention are accomplished, generally, by a system which utilizes three equally-spaced detector gates following a burst of fast neutrons and which produces data proportional to ##EQU6## where B is a function of